(1) Field of the Invention
This invention relates to a process of cold-working and stress-relieving ferritic stainless steels containing about 11 to about 30% chromium of non-heat hardenable type by extrusion and/or die drawing to produce bar, rod, wire, strip and special shapes having acceptable ductility at tensile strength levels substantially greater than that of the hot-worked and annealed starting material. Steels which may be treated by the process of the present invention include AISI Types 400, 409, 4l0-low carbon + low nitrogen, 429, 430, 433, 434, 435, 436, 442 and 446.
(2) Description of the Prior Art
Conventional annealing practices for ferritic stainless steels are intended to condition the steel for additional reduction in thickness and/or cross-section, i.e., to develop a capability for further cold-work processing. The degree of cold reduction is based on starting thickness and/or cross-section relative to the final gauge, with no predetermined or desired level of maximum mechanical strength in any stage. In other words, prior art processing is intended to produce an annealed final product which can thereafter be shaped. The mechanical strength of the shaped product approximates that of the starting material in the annealed condition.
U.S. Pat. No. 3,141,800, issued July 21, 1964 to A. T. Reichenbach, discloses a method of producing dimensionally stable plates of ferritic stainless steel, wherein hot rolled, annealed and pickled blanks are cold reduced about 18 to about 35% in thickness to final gauge, annealed between about 1350.degree. and 1450.degree. F., pickled, flattened by stretch leveling, roller leveling or temper rolling, and stress relieved by heating between 300.degree. and 800.degree. F. The final stress relief treatment is stated to be critical in order to avoid shrinkage resulting from cyclic high pressure loading at elevated temperatures. Apparently the stress relief removes some directionality in mechanical properties resulting from the flattening of the plates.
U.S. Pat. No. 3,490,956, issued Jan. 20, 1970 to J. W. Wilton, discloses a process for reducing ribbing and roping of ferritic stainless steels when subjected to a deep drawing operation. A hot rolled, annealed and pickled ferritic steel is initially cold reduced from 40 to 80% in thickness, annealed at 1600.degree. to 2100.degree. F. in a protective atmosphere, cold reduced to final thickness, and annealed at 1450.degree. to 1600.degree. F. in a protective atmosphere. Typical ultimate tensile strengths for material processed in accordance with this patent ranged from 81.5 to 86.4 ksi, with elongations ranging from 23 to 25.5%. Typically the tensile strength of a hot rolled and annealed ferritic stainless steel of the type disclosed in this patent would be about 80 to 85 ksi. Hence, the ultimate tensile strength of the final product is substantially the same as that of the hot reduced and annealed material.
U.S. Pat. No. 3,694,271, issued Sept. 26, 1972 to L.O. Egnell discloses a method of producing a composite article consisting of a supporting layer of austenitic stainless steel bonded at least on one side to an outer layer of ferritic stainless steel. A composite billet is cold reduced, e.g. by drawing, rolling and/or bending, with a reduction in cross sectional area of 5 to 70%, and subjected to an anneal at 650.degree. to 950.degree. C. (1202.degree. to 1742.degree. F.) in order to recrystallize the ferrite layer. The extent of cold reduction of each layer of the composite is not defined, and the heat treatment affects only the ferritic steel portion. It is stated that the cold working increases the yield strength and creep strength of the austenitic portion drastically, and the subsequent heat treatment is intentionally controlled in order to avoid reducing the increased mechanical strength imparted to the austenitic portion by cold working. Accordingly, the austenitic portion would not possess sufficient ductility to permit further cold reduction under practicable processing conditions.
A number of earlier patents relate to the problem of preventing roping, ribbing and/or ridging in the deep drawing of ferritic steels. In general these patents disclose that composition and heat treatment of the hot reduced product are important and that annealing of the cold worked product at temperatures of about 1350.degree. to about 1550.degree. F. is necessary in order to obtain good ductility. Representative prior art disclosures of this type include U.S. Pat. Nos. 2,772,992, issued Dec. 4, 1956 to G.C. Kiefer et al.; 2,808,353, issued Oct. 1, 1957 to W.B. Leffingwell et al.; 2,851,384, issued Sept. 9, 1958 to J.H. Waxweiler; 3,067,072, issued Dec. 4, 1962 to W.B. Leffingwell et al.; and 3,128,211, issued Apr. 7, 1964 to J.H. Waxweiler.
To the best of applicant's knowledge, the prior art neither discloses nor suggests the possibility of substantially increasing the mechanical strength of non-heat hardenable ferritic stainless steels while at the same time retaining sufficient ductility to permit subsequent cold-work forming, e.g. cold heading operations and production of spring-temper wire.
There is a definite need for ferritic stainless steels having an ultimate tensile strength up to about 310 ksi along with 18% tensile ductility, cold headability at ultimate tensile strengths of about 120 to about 140 ksi, and spring-temper characteristics equivalent to those of AISI Type 302 spring wire, for application in products such as automotive thermostat springs, windshield wiper arms, automotive fasteners and straight pins.